Falling film Distributors

The monoglycerides of the raffinate (the bottom product) is shown in Figure 9 as a function of the superficial velocity of the gas phase at a phase ratio of 19. At an superficial velocity of 10 mm/s the raffinate obtained with different packings is nearly the same except for the Sulzer Packing SMV. At lower superficial velocities the wire mesh packings (Sulzer CY) provide the best yields. The experiments are made at conditions where a falling film disintegrates into drops. Therefore, it seems understandable that the efficiency of the spray column and that of the collector-distributor installations do not much differ. 

Figure 10. Tube distributors for falling film evaporators. (Henry Vogt Machine Company.)...

The chief problem in a falling-film evaporator is that of distributing the liquid uniformly as a film inside the tubes. This is done by a set of perforated metal plates above a carefully leveled tube sheet, by inserts in the tube ends to cause the liquid to flow evenly into each tube, or by spider distributors with radial arms from which the feed is sprayed at a steady rate on the inside surface of each tube. Still another way is to use an individual spray nozzle inside each tube. 

The main problem associated with falling film units is the need to distribute the liquid evenly to all tubes. All tubes must be wetted uniformly and this may require recirculation of the liquid unless the ratio of feed to evaporation is relatively high. Recirculation can only be accomplished by pumping. Distribution can be achieved with distributors for individual tubes, with orifice plates above the tubes and tubesheet, or by spraying. Updraft operation complicates the liquid distribution. 

ODC d, made of silver and PTFE (Bayer Material Science AG, BMS), within the so-called percolator c, a woven fabric which adjusts the flow rate of caustic soda solution. The ODC d and the cathodic current distributor f are electrically connected on their entire area by the elastic element e, made of fine, interwoven nickel wire. This elastic element e presses the ODC d, the percolator c, and the membrane b with optimized pressure onto the anode a. Therefore, the usual anode half shell of the conventional membrane process, completely filled with anolyte, can be apphed, and no falling film construction is necessary on the anode side as it is used in [10]. The percolator c withstands the pressure of the elastic element e and remains sufficiently permeable for the catholyte flow. Free oxygen gas transport into the ODC d is possible through the elastic element e. 

In gas-liquid spray towers the liquid is atomized and enters as a fine spray at the top and the gas is introduced at the bottom. The gas flow rate has to be kept sufficiently low to permit the liquid to fall. It is generally chosen in such way that the liquid drops of mean diameter fall at 20 percent of their free-fall velocity, as calculated from Stokes law. An efficient dispersion of the liquid requires the openings of the distributor to be small and the pressure high. Thereby a fraction of the drops hits the wall and flows down the wall as a film. Furthermore, a certain degree of coalescence of the drops is inevitable, so that the drop size, velocity, and therefore residence time vary strongly with position. A rigorous hydrodynamic analysis of such a situation is extremely complicated so that only the overall behavior has been studied. 

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